1. Field of the Invention
The present invention relates to a resin composition which has such performances as excellent rubber elasticity, favorable oil maintainability, low hardness and low compression set, has excellent adhesivity to a metal, and is well suited as a gasket material for use in a gasket integrated with a metal wherein a gasket is integrated into a metallic cover body or a metallic frame body.
In addition, the present invention pertains to a composite molded body in which a metallic sheet and a thermoplastic elastomer are integrated with each other, more particularly to a composite molded body in which a metallic sheet and a thermoplastic elastomer are integrated with each other without being subjected to surface treatment or without the use of an adhesive and which is well suited for use in a gasket fitted to a cover, etc. that is employed for housing electronic equipment and the like.
Moreover, the present invention is related to a process for producing a composite molded body of a resin and metal, for instance, a gasket integrated with a metal which has excellent adhesivity between a metallic part and a resin portion formed by injection molding of a thermoplastic elastomer composition.
Further, the present invention is concerned with a process for producing a gasket integrated with a cover having excellent gasket performance by injection molding of a thermoplastic elastomer composition.
2. Description of the Related Arts
In recent years, electronic equipment such as a computer has come to have intricate circuit constitution, accompanying the progress towards a high performance and compactification, and is prone to be readily damaged even by a slight amount of dust. Accordingly, dust prevention is increasingly required in practical application, thus making it a general practice to prevent dust penetration by the use of a gasket.
The above-mentioned gasket is, in many cases, in the form of a gasket integrated with a metal in which a gasket material composed of a thermoplastic elastomer of low hardness has been injection molded onto a metallic part such as a case lid.
The methods as described hereunder have heretofore been adopted as a method for integrating a thermoplastic elastomer of low hardness with a metallic part.
{circle around (1)} a method which comprises fixing a gasket to a cover body by means of a double-coated tape;
{circle around (2)} a method which comprises sandwiching a cover body having holes between gasket materials through the holes;
{circle around (3)} a method which comprises applying in advance, an adhesive to the surface portion of a metallic part onto which an elastomer is to be injection molded or extrusion molded; and
{circle around (4)} a method which comprises making in advance, through holes or grooves on the portion of a metallic part onto which an elastomer is to be injection molded so as to latch the injection molded portion.
However, any of the above-mentioned methods is complicated in production steps, and thus needs a high production cost. As the above-mentioned method {circle around (1)} which comprises fixing a gasket to a cover body by means of a double-coated tape, there is usually adopted a method in which a sheet-like gasket material to which a double-coated tape is adhered, is punched into a shape of gasket, and the resultant gasket is fixed to the cover body. However, the aforesaid method is involved in such problems that the production process is made intricate or troublesome, thus constituting a factor increasing the production cost, since most of the sheet-like gasket material after punching remains as waste material and besides, the gasket is fixed to the cover body after the gasket material has been punched into a shape of gasket.
In addition, in the gasket fitted to a cover body which is produced by the above-mentioned method {circle around (2)} which comprises sandwiching a cover body having holes between gasket materials through the holes, since the gasket material is exposed on the upper side of the cover body and besides is insufficiently adhered to the cover body, the gasket material exposed thereon is sometimes upturned or curled when the cover body is inserted into the space of an electronic equipment main body, which space has been narrowed accompanying the recently compactified electronic equipment part items. Thus, there is caused the problem that such upturning or curling is responsible for defective sealing properties.
Moreover, in the case where the gasket which is fitted to a cover body and which is produced by such a method is used in a hard disc drive apparatus, there is caused the problem that it is impossible to suppress the vibration due to the rotation of the hard disc.
In general, the adhesion between a thermoplastic elastomer and a metallic sheet is carried out by a method in which the metallic sheet is subjected to a surface treatment, and the treated surface is coated with an adhesive to be adhered to the thermoplastic elastomer. In order to enhance the adhesivity, there is also carried out blending of an adhesive (a pressure sensitive adhesive) material in the thermoplastic elastomer.
However, the adhesion between the thermoplastic elastomer and the metallic sheet by the use of an adhesive is involved in such problems that its production process becomes intricate and troublesome, constituting a factor increasing the production cost, since the process requires the steps of surface treatment for a metallic sheet and adhesive coating.
In the case of using, in precision electronic equipment and the like, a molded body in the form of a gasket fitted to a cover body wherein a thermoplastic elastomer is adhered to a metallic sheet, there is caused a problem of gas generation due to the surface treatment and adhesive coating, thereby making it difficult to use the precision electronic equipment. Further in the case of blending an adhesive (a pressure sensitive adhesive) material in a thermoplastic elastomer, there are caused such problems in that the tacking (pressure sensitive self adhesion) imparted to the resultant blend makes handling itself extremely difficult and also in that the blending of the adhesive (a pressure sensitive adhesive) material deteriorates the physical properties such as the compression set of a thermoplastic elastomer, whereby the elastomer no longer functions as a gasket material.
On the one hand, as a method for laminating a thermoplastic resin on a metallic sheet, there is described in Japanese Patent Application Laid Open Nos. 235783/1999 (Heisei 11) and 262978/1999 (Heisei 11), an integration method in which a polyolefin film side, that is, the resin side is subjected to a corona treatment and laminated with a metallic sheet. Methods for integrally molding a styrenic thermoplastic elastomer and poly-propylene by injection molding are described in Japanese Patent Application Laid Open Nos. 139232/1990 (Heisei 2), 99331/1996 (Heisei 8) and 266375/1995 (Heisei 7).
Nevertheless, according to single use of any of the above-stated methods, the resultant molded body does not function at all as a gasket fitted to a cover body. Saying it differently, a metallic sheet laminated with a thermoplastic resin film, even if the film thickness is extremely increased, does not function at all as a gasket by the single use of a rigid resin film. In addition, it is absolutely impossible to expect that a composite material formed by integrally injection molding a styrenic elastomer and polypropylene will function as a cover body because of the low rigidity of polypropylene as the substrate.
In the case of producing a gasket integrated with a metal by a method wherein a gasket portion is formed by injection molding or extrusion molding a thermoplastic elastomer onto the surface of a metallic cover body, the direction of the injection molding or extrusion molding for the thermoplastic elastomer has usually been the direction towards a butt contact surface on the gasket portion upon the use thereof (the surface of the gasket for the purpose of realizing hermetical sealing by coming into butt contact with a vessel body at the time of its use; hereinafter sometimes referred to simply as xe2x80x9cbutt contact surfacexe2x80x9d).
However, the following disadvantages have been involved in the above-mentioned case.
{circle around (1)} When there is installed a spout for injection molding or extrusion molding a thermoplastic elastomer separately from the gasket portion, a space therefor is made necessary, thus causing restriction on gasket design.
{circle around (2)} When there is installed a spout on the butt contact surface for the time of its use to avoid the space problem, there is a fear of losing flatness of the gasket and causing defect of gasket performance.
{circle around (3)} When an attempt is made to dispose a gasket rubber portion as close as the end of a metallic cover surface, it is necessary to locate an injection molding port also as close as the end. However, such location is difficult from the aspect of design, since in many cases, the end portion of a metallic cover is bent to the gasket location side for the purpose of assuring the rigidity of the cover. Saying it differently, the injection molding port of an injection molding machine interferes with the bending part of the metallic cover end. Therefore, the gasket rubber portion can not easily be located as close as the end portion of a metallic cover surface.
Under such circumstances, it is a general object of the present invention to provide a resin composition which has such performances that are required as a gasket material including excellent rubber elasticity, favorable oil maintainability, low hardness, excellent seating properties and low compression set, which has excellent adhesivity to a metal, and which is well suited as a gasket material for use in a gasket integrated with a metal.
It is another object of the present invention to provide a composite molded body which can be produced dispensing with an intricate or troublesome step, which is excellent in vibration damping properties, and which is well suited for use in a gasket fitted to a cover body.
It is still another object of the present invention to provide a process for producing a composite molded body of a resin and metal in which the resin portion and the metallic portion are sufficiently adhesively fixed to each other not through an intricate production step.
It is a further object of the present invention to provide a process for producing a gasket integrated with a metal so as not to cause a defect of gasket performance due to restriction on the gasket design or the loss of flatness of a gasket face.
Other objects of the present invention will be obvious from the text of the specification hereinafter disclosed.
A proposal was previously made by the present inventors on a gasket material which is composed of a hydrogenated block copolymer, a non-aromatic rubber-softening agent and a polyolefinic resin each having specific physical properties at specific proportions, and which has excellent rubber elasticity, good oil maintainability, low hardness, excellent seating properties and low compression set {Japanese Patent Application Laid Open No. 344879/1999 (Heisei 11)}. In addition, intensive extensive research and investigation were accumulated by the present inventors in order to attain the above-mentioned objects. As a result, it has been found that the objects thereof can be achieved by combinationally employing the hydrogenated block copolymer in the above-mentioned invention as a prior art and a modified hydrogenated block copolymer.
It has also been found that the objects thereof can be achieved by the composite molded body which is formed by laminating a metallic sheet with a specific thermoplastic resin, and then integrating a specific thermoplastic elastomer with the laminate thus obtained.
It has further been found that the objects thereof can be achieved by subjecting part or all of the surface of a metallic part that comes into contact with a resin portion to a corona treatment or a plasma treatment, and thereafter placing the metal-resin laminate in a mold.
It has still further been found that the objects thereof can be achieved by injection casting a thermoplastic elastomer composition into a mold from the direction different from the butt contact surface of a gasket portion. It being so, the present invention has been accomplished on the basis of the above-mentioned findings and information.
Specifically, the present invention provides a resin composition which comprises a thermoplastic elastomer composition comprising 100 parts by weight of a (a1) hydrogenated block copolymer which is obtained by hydrogenating a block copolymer consisting essentially of at least one polymer block composed principally of a vinyl aromatic compound and at least one polymer block composed principally of a conjugated diene compound, which has a weight average molecular weight of at least 200,000, in which the at least one polymer block composed principally of a vinyl aromatic compound is polystyrene, and in which the content by weight of the polystyrene moiety that is contained in the hydrogenated block copolymer is in the range of 20 to 40% 100 to 200 parts by weight of a (b1) non-aromatic rubber-softening agent having a kinematic viscosity at 40xc2x0 C. of at least 300 mm2sxe2x88x921; and 10 to 50 parts by weight of a (c1) modified polyolefinic resin, said resin composition having a hardness as measured in accordance with JIS K6253 with a durometer of type A being at most 50 degrees and a compression set as measured in accordance with JIS K6262 after being allowed to stand at 70xc2x0 C. under a compression ratio of 25% for 22 hours being at most 50%.
Moreover, the present invention provides a gasket material composed of the above-mentioned resin composition according to the present invention; a gasket integrated with a metal formed by molding by the use of the aforesaid gasket material; and a hard disc gasket composed of the gasket integrated with a metal as mentioned above.
In addition, the present invention provides a thermoplastic elastomer composition comprising 100 parts by weight of a (a2) hydrogenated block copolymer which is obtained by hydrogenating a block copolymer consisting essentially of at least one polymer block composed principally of a vinyl aromatic compound and at least one polymer block composed principally of a conjugated diene compound, which has a weight average molecular weight of at least 200,000, in which the at least one polymer block composed principally of a vinyl aromatic compound is polystyrene and in which the content by weight of the polystyrene moiety that is contained in the hydrogenated block copolymer is in the range of 20 to 40%; 100 to 200 parts by weight of a (b2) non-aromatic rubber-softening agent having a kinematic viscosity at 40xc2x0 C. of at least 300 mm2sxe2x88x921; 10 to 50 parts by weight of a (c2) modified hydrogenated block copolymer obtained by hydrogenating a block copolymer in which a functional group is imparted to a block copolymer consisting essentially of at least one polymer block composed principally of a vinyl aromatic compound and at least one polymer block composed principally of a conjugated diene compound; and 0 to 20 parts by weight of a (d2) compound composed principally of a crystalline polyolefin, said thermoplastic elastomer composition having a hardness as measured in accordance with JIS K6253 with a durometer of type A being at most 50 degrees and a compression set as measured in accordance with JIS K6262 after being allowed to stand at 70xc2x0 C. under a compression ratio of 25% for 22 hours being at most 50%.
Also, the present invention provides a composite molded body that is formed by integrating through thermally fusing adhesion, a metallic sheet laminated with a thermoplastic resin having a hardness as measured in accordance with JIS K6253 with a durometer of type A being at least 50 degrees and a thermoplastic elastomer composition having a hardness as measured in accordance with JIS K6253 with a durometer of type A being at most 60 degrees and a compression set as measured in accordance with JIS K6262 after being allowed to stand at 70xc2x0 C. under a compression ratio of 25% for 22 hours being at most 50%.
Further, the present invention provides a gasket material composed of the above-stated thermoplastic elastomer composition according to the present invention; and a gasket integrated with a metal formed by molding the aforesaid gasket material into a gasket integrated with a metallic cover or a gasket integrated with a metallic frame.
Still further, the present invention provides a process for producing a composite molded body of a resin and metal which comprises subjecting part or all of the surface of a metallic part that is supposed to come into contact with a resin portion to a corona treatment or a plasma treatment, thereafter placing the metallic part in a mold, and then injecting a thermoplastic elastomer composition onto the surfaces of the metallic part placed in the mold to form a resin portion.
Aside therefrom, an attempt to subject a resin side to a corona treatment for the purpose of enhancing the adhesivity to a metal is described, for instance, in Japanese Patent Application Laid Open Nos. 235787/1999 (Heisei 11) and 262978/1999 (Heisei 11), but a corona treatment or plasma treatment of a metal side has never hitherto been attempted at all.
Furthermore, the present invention provides a process for producing a gasket integrated with a cover body by placing a metallic cover body in a mold, and injecting a gasket material composed of a thermoplastic elastomer composition onto the surfaces of said cover body so as to form a gasket portion, characterized in that the thermoplastic elastomer composition is injection cast from the direction different from the direction towards a butt contact surface on the gasket portion upon the use thereof.